High energy CRM produced in the atmosphere impinge on the earth's surface and penetrate deep below the surface. The flux and angular distribution of these particles at the surface is well known and their flux and angular distributions at various depths below the surface has been studied for standard geological situations. The penetration depth into the earth of any single particle of given energy at the surface will depend on the amount and composition of the material it encounters along its trajectory.
Measurements of the flux of CRM along well defined directions near the earth's surface have been employed to uncover evidence of undiscovered cavities in pyramids, L. W. Alvarez et al., Science 167 (1970) 832 (“Alvarez”), and low density regions in volcanoes, K. Nagamine et al., NIM A356 (1995) 358 (“Nagamine”) and H. Tanaka, et al., NIM A507 (2003) 657 (“Tanaka”), the disclosures of which are hereby incorporated herein, in their entirety, by reference. Because the intensity of CRMs along a particular direction between a massive object and a detector is directly related to the total amount and composition of the material (X) through which the CRM has passed before reaching the detector, by observing the CRMs passing through the same object at one or more angles, a density image of the object such as a volcano may be formed.
The techniques described by Alvarez, Nagamine, and Tanaka are similar in some respects to those employed in medical and industrial imaging such as computed tomography (CT), as described in, for example, in K. Klingenbeck-Regn et al., “Subsecond multi-slice computed tomography: basics and applications,” Eur. J. Radiol. 31 (1999), 110-124 (“Klingenbeck-Regn”), the disclosure of which is hereby incorporated herein, in its entirety, by reference. These techniques use the differential absorption of particles (x-rays in the case of CT) along various lines of sight are used to construct image “slices” reflecting variations within the internal structure of the patient or object under study.
The measurement techniques utilized by Alvarez, Nagamine, and Tanaka employed position and angle measuring CRM detector systems positioned near the surface of the earth to obtain crude images of objects above the detectors for the detection of low density regions within the objects, e.g., pyramids and volcanoes, under study. The distributions of CRM which penetrated the object as determined by the intensities n(θ) at various and angles (θ), were analyzed to produce crude density image projections since there is a unique relationship between n(θ) and the amount of material X(θ) along the direction measured.
Thus, regions of lower density could be revealed by measuring n(θ) for a given angle θ to determine X(θ). In the cases described by Nagamine and Tanaka, nearly horizontal CRM passing through the volcano were compared to those passing through the atmosphere to locate regions of low density in a known direction. In the case described by Alvarez, nearly vertical CRM were used to survey the structure of the pyramid above the detectors.